One of the most common and frequently essential image processing operations is noise reduction. This is especially true for digital still and video camera images that may have been captured under insufficient lighting conditions. One way to address digital image capture under less than optimum lighting conditions is to either acquire or synthesize one or more color channels that are particularly sensitive to low or insufficient scene illumination. The data from the channels with increased light sensitivity are generally used to guide the subsequent image processing of the data from the accompanying standard color channels. Noise reduction is a prime candidate for benefiting from this additional image data. A number of examples exist in the literature. U.S. Pat. No. 6,646,246 (Gindele, et al.) teaches using an extended dynamic range color filter array (CFA) pattern with slow and fast pixels, noise-cleaning the slow pixel data using only slow pixel data and noise-cleaning the fast pixel data using only fast pixel data. This approach achieved noise reduction at the expense of image resolution as each color channel is now subdivided into a fast channel and a slow channel and the subsequent merger can produce image processing artifacts more troublesome than the addressed original noise. U.S. Pat. No. 7,065,246 (Xiaomang, et al.) is representative of a fair number of similarly disclosed inventions in that it reveals constructing a luminance signal from directly sensed color channel data, in this case cyan, magenta, yellow, and green. The high-frequency component of the constructed luminance is used to replace the high-frequency component of the original color channel signals to affect a net noise reduction of the image data. Although somewhat effective, the major liability of this approach is that the synthesized luminance channel is constructed from noisy color channel components resulting in an essentially equally noisy synthetic channel.
A suggestion of a better approach can be found in U.S. Pat. No. 5,264,924 (Cok). Cok discloses direct measurement of red, green, blue, and luminance values at each pixel location. The high-frequency luminance data which is designed to be inherently less noisy than the corresponding high-frequency red, green, and blue data is used to replace said high-frequency red, green, and blue data to produce noise-cleaned red, green, and blue signals. Since the vast majority of digital still and video cameras use a single sensor equipped with a CFA that only senses one color channel per pixel, Cok cannot be directly practiced in such systems.
Although Xiaomang and Cok describe luminance signals, a color channel with photometric sensitivity conforming to the luminance channel of the human visual system may be unnecessarily restrictive.